Pollen morphology sheds light on the systematics of the tribes Palicoureeae and Psychotrieae (Rubiaceae)

This is the authors' abstract. We don't add key points for this paper.

Pollen morphology has been an important source of taxonomic information in angiosperms, with Rubiaceae showing remarkable diversity. This study analyzes species from the tribes Palicoureeae and Psychotrieae, which have undergone taxonomic rearrangements to ensure their monophyly. The sample comprises 34 species in total of Eumachia (2 species), Palicourea (16 species), Psychotria (12 species), Rudgea (3 species), and Geophila repens. Acetolyzed pollen grains were analyzed under light microscopy, measured and photographed. Additionally, non-acetolyzed pollen grains were analyzed via scanning electron microscopy. Pollen data were analyzed using multivariate methods. Two species groups were recognized based on pollen polarity. Exine ornamentation and aperture type, or its absence, were the most variable pollen characteristics. While a reticulate-heterobrocate exine pattern predominates, foveolate, rugulate, granulate, perforate, scabrate, microreticulate-echinate, and microreticulate types were also observed. Cluster analysis revealed three major groups, encompassing species within both Psychotria and Palicourea, all characterized by apolar, inaperturate pollen grains. Isopolar pollen grains were found in two species of Eumachia, two of Psychotria and one of Rudgea. Pollen apertures emerge as an informative feature for circumscribing Palicourea. The results reveal a predominance of inaperturate pollen grains in Palicourea, even among species still circumscribed within Psychotria, supporting the taxonomic transfer of these Psychotria species to Palicourea. Once future phylogenetic studies, including of the species analyzed here, become available, it will be possible to test hypotheses regarding the evolutionary significance of this pollen trait and investigate the role of exine ornamentation in the infrageneric classification of Palicourea.